Diacetyl, a reactive diketone used in artificial butter flavoring has been associated with obliterative bronchiolitis (OB) in employees at microwave popcorn packaging plants; however, the mechanisms of toxicity are unknown. We recently demonstrated that diacetyl, the major volatile component of artificial butter flavoring, caused OB-like lesions in rats after inhalation exposure. Because of concerns about diacetyl toxicity, it is being replaced in some consumer products by 2,3-pentanedione (PD), and possibly 2,3-hexanedione (HD), both structurally-related and untested chemicals. Because the toxicity of inhaled PD and HD are unknown, studies were conducted to characterize the toxicity after inhalation exposure to a range of concentrations in rodents. Male and female Wistar-Han rats were exposed to 0, 50, 100, or 200 ppm PD 6h/d, 5d/wk for up to 2 wk. HD caused only minor epithelial changes at the highest concentration; owever, PD was found to cause OB-like lesions similar to those caused by diacetyl. The epithelium lining the respiratory tract was the site of toxicity for all three chemicals. Diacetyl and PD both caused fibrotic airway lesions in rats with pathological features of OB. The histopathological and biological changes observed in rats indicate that PD is not an acceptable replacement for diacetyl. We sought to evaluate changes in gene expression in the distal bronchi of rats with PD-induced OB. Male Wistar Han rats were exposed to 200 ppm PD or air (controls) 6 h/d, 5 d/wk for 2-wks. Bronchial tissues were laser microdissected from serial sections of frozen lung. In exposed lungs, both fibrotic and non-fibrotic airways were collected. Following RNA extraction and microarray analysis, differential gene expression was evaluated. In non-fibrotic bronchi of exposed rats, 1548 genes were significantly altered relative to air-exposed controls with notable down-regulation of many inflammatory cytokines and chemokines. In contrast, in PD-exposed fibrotic bronchi, 2504 genes were significantly altered with a majority of genes being up-regulated in affected pathways. Tgf-2 and downstream genes implicated in fibrosis were significantly up-regulated in fibrotic lesions. Genes for collagens and extracellular matrix proteins were highly up-regulated. In addition, expression of genes for peptidases and for peptidase inhibitors were significantly altered suggesting tissue remodeling that may contribute to fibrosis. Our data provide new insights into the molecular mechanisms of OB, and illustrate that PD-induced OB shares pathologic features with other fibrotic lung diseases. This new information is of potential significance with regards to future therapeutic targets for treatment.